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e June 1990 e Tyus and Nikirk-Abundance, growth, and diet of Ieta/urus punctatus 189 our findings relative to the potentia] effect of channel catfish on recovery of endangered Col- orado River fishes. MATERIALS AND METHODs-Channel catfish were captured by U.S. Fish and Wildlife Service personnel during 1987 and 1988 using electrofishing"eining, and angling techniques. A sample of catfish was obtained in each of seven reaches (Fig. 1) between River Ki- lometer (RK) 35 and 552 of the Green River, and one reach (0-74 km) of the Yampa River in Dinosaur National Monument. These reaches were selected for differences in habitat types, including low-I~radient riv- er reaches (average drop in water surface elevation = 0.2 to 0.4 m/km) predominated by sand and silt sub- strates (A, B, E, and F in Fig. 1) versus higher gradient reaches (1.3 to 2.1 m/km drop in elevation) predom- inated by boulder and cobble substrates (reaches C, D, and G and Yampa River in Dinosaur National Mon- ument; Fig. 1). The relative abundance of adult chan- nel catfish was evaluated by spring sampling of shore- line habitats and involved continuous downstream coverage with a pulsed DC unit. Angling with cricket and worm-baited hooks was also conducted in July and August at selected eddy habitats in lower Yampa River, Desolation and Gray canyons, and in strata E and F (Fig. 1). All catfish were sacrificed, and total length in mm (TL) and weight in g were recorded. Total length data allowed ease of comparison with other studies (see Carlander, 1969). One or both pectoral spines were removed at the time of capture for aging. The dried spines were cleaned of all skin and dried flesh and mounted in clear casting resin for sectioning. Mounted spines were sectioned using an apparatus similar to that of Margenau (1982), except that a dremel tool with metal, cutting blades or a jewelers' hand saw was used to cut the sections. Sections were cut at the distal end of the basal recess in an attempt to include all annuli and examined under 25 x magnification. Spines were read for the presence of annular rings using meth- ods of Sneed (1951) and Marzolf (1952). Two tech- nicians inspected each section independently to deter- mine the age of each fish, and spine sections were also sent to another investigator for aging. Number of an- nuli reported by an independent investigator (R. A. Whaley, pers. comm.) agreed with about 90% of the annuli detected by our technicians. Sections were polished by hand using very fine (#600 grit) sandpaper, placed in toluene for clearing, and an image projected (Ken-a-vision model X-lOOO-l) at 25 to 40 x magnification. Annular radii in each section were measured on a line from the focus to the edge of the longest lobe. The edge of the spine was considered an annulus on fish collected in the non-growing season, (i.e., between October 1 and May 31), and the date of annulus formation was assumed to be 1 January by convention (Jearld, 1983). FIG. I-Location of study areas, Green and Yampa rivers, Colorado and Utah. Total lengths at consecutive annuli were back-cal- culated using the formula L' = (S' /S)L where L' = TL at annulus formation, S' = annular radius, S = total spine radius, and L = TL at time of capture. Length-weight relationships were expressed as simple linear regressions, and condition factor (K) was cal- culated as WT x 105/TU (Ricker, 1975). Data ob- tained by a U.S. Fish and Wildlife Service sampling program in the same area from 1979 to 1985 were used to determine the maximum total length attained by channel catfish during their first growing season and to verify our aging of the age 0 life history stage. Because the lumen of the pectoral spine may erode the first and possibly also the second annulus in spines of older individuals (Sneed, 1951; Marzolf, 1952, 1955), the maximum length attained by channel catfish in one growing season in the Green River was used to deter- mine if the first annulus was present. If it was judged that the first annulus was not present (the back-cal- culated length at age 1 exceeded the 80 mm maximum attainable length for one growing season), 1 year was added to the age of the fish, and back-calculated lengths at consecutive annuli were shifted 1 year to reflect the true annual growth. We used sections exhibiting a true, first annulus to back-calculate the maximum length at the second annulus. If the back-calculated length at the second annulus exceeded this length (150 mm), it was assumed that the second annulus had been eroded by